Watlow ez-zone pm - Proportional and (P) Control; Proportional and Integral (PI) Control

Watlow EZ-ZONE
®
PMI Controller • 195 • Chapter 9 Features
control can be selected with Heat Algorithm h.ag or Cool Algorithm C.ag (Setup Page, Loop
Menu). On-off hysteresis can be set with On/Off Heat Hysteresis h.hy or On/Off Cool Hyster-
esis C.hy (Operations Page, Loop Menu).
Note:
Input Error Failure Mode faiL does not function in on-off control mode. The output
goes off.
Proportional and (P) Control
Some processes need to maintain a temperature or process value closer to the set point than
on-off control can provide. Proportional control provides closer control by adjusting the
output when the temperature or process value is within a proportional band. When the value
is in the band, the controller adjusts the output based on how close the process value is to
the set point. The closer the process value is to the set point, the lower the output power.
This is similar to backing off on the gas pedal
of a car as you approach a stop sign. It keeps
the temperature or process value from swing-
ing as widely as it would with simple on-off
control. However, when the system settles
down, the temperature or process value
tends to “droop” short of the set point.
With proportional control, the output power
level equals the set point minus the process
value divided by proportional band times 100.
In an application with one output assigned
to heating and another assigned to cooling,
each will have a separate proportional parameter. The heating parameter takes effect when
the process temperature is lower than the set point, and the cooling parameter takes effect
when the process temperature is higher than the set point.
Adjust the proportional band with Heat Proportional Band h.pb or Cool Proportional Band
C.pb (Operations Page, Loop Menu).
Proportional and Integral (PI) Control
The droop caused by proportional control can be corrected by adding integral (reset) con-
trol. When the system settles down, the integral value is tuned to bring the temperature or
process value closer to the set point. Integral determines the speed of the correction, but
this may increase the overshoot at startup or when the set point is changed. Too much inte-
gral action will make the system unstable. Adjust the integral with Time Integral ti (Opera-
tions Page, Loop Menu).
Time
Temperature
On/Off System Cycles
The cooling action
switches
on at startup.
Process Temperature
The cooling action switches on when
the process temperature rises above
the set point plus the hysteresis.
The cooling action switches off when the process
temperature drops below the set point.
Set Point
Time
Temperature
The heating action switches off when the process
temperature rises above the set point.
The heating action
switches on at startup.
On/Off Hysteresis
The heating action switches on when the process temperature
drops below the set point minus the hysteresis.
On/Off System Cycles
Set Point
Time
Temperature
Proportional Control
Set Point
Proportional Band
Droop
Overshoot